1. Field of the Invention
The present invention relates to a method of forming a polyimide-isolating wall, more particularly, to a method of forming a polyimide-isolating wall with a reverse-trapezoid cross-section that possesses electric, thermal and mechanical stability.
2. Description of Related Art
Currently, an organic electroluminescent display (OEL) panel with light weight, high contrast, wide view angle, fast response speed, low power consumption and high brightness has become the focus of the innovative display panels. However, there are many difficulties to be overcome in the commercialization and mass production of an OEL panel.
Generally, the OEL panel comprises a plurality of ITO anodes, a plurality of photoresist-isolating walls perpendicularly positioned on the anodes, an OEL layer positioned between two adjacent photoresist isolating walls on the anodes, and a plurality of cathodes positioned on the OEL layer. The anodes and the cathodes sandwich the OEL layer to form a luminescent pixel of the OEL panel. However, the ordinary photoresist materials often release out the residual solvents or moisture to the sealed space of the OEL panel. The released residual solvents or moisture further damage the OEL layer and thereby reduce the quality and lifetime of the panel.
In the technology disclosed in U.S. Pat. Nos. 5,701,055 and 5,952,037, a photoresist (OFPR-8000 offered by Tokyo Ohka Co. ltd) with unsatisfied electric, thermal and mechanical stability is taught to act as isolating walls to isolate anodes and cathode materials. The electric thermal and mechanical stability of the photoresists disclosed in U.S. Pat. Nos. 5,701,055 and 5,952,037 is not good enough and often causes bad Iminescent stability and failure of the panel. On the other hand, the luminescence of the OEL panel is driven by a high electric current. The high electric currents and frequent switch operations of the OEL panel further deteriorates the photoresist isolating walls by inducing continuously liberation of harmful substances such as residual moisture and solvents. The degradation of the isolating walls will bring about a short circuit or failure in luminescence and reduce the lifetime of the OEL panel.
In addition, the photoresist isolating walls having approximately rectangular cross-section were obtained after performing the exposure process by a mask and the developing process on the ordinary photoresist material. The cathod materials diffuse laterally and thereby possibly contact the anodes during the follow-up deposition of the cathode materials. This will also lead to a short circuit on the panel. Though a multi-layer structure with reverse-trapezoid section has been previously disclosed in published patents, the procedure of fabricating multi-layer isolating walls is complicated and expensive. The process of multi-layer isolating walls is not feasible in the mass production of the OEL panel.
The object of the present invention is to provide a method of forming a polyimide-isolating wall with a reverse-trapezoid cross-section that possesses electric, thermal and mechanical stability and thereby improves the quality and lifetime of an OEL display panel.
In a preferred embodiment, the present invention provides a method of forming a polyimide-isolating wall with a reverse-trapezoid section on an OEL display panel. The panel comprises a substrate and a plurality of first electrodes. In the present invention, a coating process is firstly performed to coat a photosensitive-polyimide (PSPI) isolating layer on the panel. Then, a prebaking process, an exposure process, a post-exposure baking (PEB) process and a developing process are performed on the isolating layer in sequence to form the polyimide-isolating wall with the reverse-trapezoid cross-section. Finally, a high temperature curing process is performed on the polyimide-isolating wall with the reverse-trapezoid cross-section to ensure electric, thermal and mechanical stability that thereby improves the stability and lifetime of the OEL display panel.
The OEL display panel can be applied to any purpose or apparatus for displaying images, pictures, symbols and characters. The preferred purpose is used as a display of television, computer, printer, screen, vehicle, signal, communication apparatus, telephone, interactive electronic book, microdisplay, fishing apparatus, personal digital assistant, game, airplane and game goggle.
It is an advantage of the present invention that the method of forming the polyimide-isolating wall with the reverse-trapezoid cross-section can isolate cathode materials and anodes and meet the mass production requirements. The polyimide-isolating wall with the reverse-trapezoid cross-section provides with good comparability and interface adhesion with the under pixel-defining layer of polymide. Also, the polyimide-isolating wall with electric, thermal and mechanical stability can ensure the luminescent stability of the OEL panel.
Other objects, advantages, and novel features of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.